The primary goal of the 1.6.x series is to round out our 1.9 support by fixing any reported incompatibilities. Of course, as with any JRuby release, we will continue fixing any found incompatibilities and also improve performance. All users of 1.6.5.1 (and lower) are encouraged to upgrade to 1.6.6.

Because master keeps getting further and further away from our 1.6 branch we have decided to make this our last 1.6 release. We largely fulfilled our goal of having reasonable 1.9 support. Follow up fixes for 1.9 support will only be fixed on master from this point forward. JRuby 1.7.0 will be the next release of JRuby.